1. Field of the Invention
This invention relates to the control of droplet-size distributions in aqueous aerial sprays or discharges and, more particularly, relates to the minimization of spray drift.
2. Description of the Prior Art
Mist, or the fine particles end of the droplet-size spectra (typically those less than 150 microns in diameter) in industrial aqueous spray or discharge processes, such as those associated with aerial firefighting and dust control, gas scrubbers, crude oil spill treatments and various bioactive ingredient application processes, particularly those associated with agriculture, often reduce the effectiveness of these processes.
When the sprays are to be directed toward a specific target, the aerial spray or discharge delivery systems are typically mounted on airplanes, tractors, ground rigs or railcars. However, as a result of spray drift, much of the material in a spray can be rendered ineffective because of the inability of the small diameter spray particles to reach and impact upon the intended target. It is well known that spray droplet-size is a major factor affecting drift. While small droplets provide better coverage of a target, they are more susceptible to drift than larger droplets. Spray drift represents a loss of chemical from intended targets and thus implies the dangers inherent in air and water pollution. Since off-target chemicals are wasted product and with agricultural sprays, in particular, can represent a hazard to surrounding crops, water supplies and livestock, spray drift is an economical and environmental concern.
Research efforts to reduce spray drift have typically dealt with improved equipment design, e.g., nozzle design to optimize spray patterns, or application techniques such as spray pressures, heights, formulations, etc. The most promising improvements in the application technology area have been in the reduction of fine spray droplets in the droplet spectrum during atomization via the use of spray modifiers known as drift control agents. Effective drift control agents must possess a great number of characteristics for they must be able to increase the small droplet size; be insensitive to the high shear process conditions realized in the spray system pumps, nozzles, etc.; not detract from the biological effects of the spray bioactives; be compatible with other spray adjuvants, i.e., non-bioactive material added to the spray mixture to improve chemical or physical characteristics; not separate upon standing; be easy to use; be environmentally friendly; and be cost efficient.
Drift control agents are usually high molecular weight polymers which, when added to aqueous systems, tend to increase the viscosity of the system and thus prevent the water from being broken up into a fine mist when aerially sprayed or discharged.
These high molecular weight polymers tend to be unstable in that they often degrade upon aging and are very shear sensitive: both of which conditions, upon occurrence, cause a decrease in solution viscosity with a concomitant decrease in drift control activity.
Typical polymers currently utilized as drift control agents are the visco-elastic polyacrylamides, the polyethylene oxides, and the poly (vinyl pyrrolidones), with the polyacrylamides being the agriculture industry spray tank additive, drift reduction standard. However, current polyacrylamide drift control spray formulations have a very limited effective time of positive drift reduction for a number of reasons. At the outset, the synthetic polyacrylamide polymer drift control agents are usually distributed in a kerosene carrier, which limits the dispersibility and additionally presents a volatile organic component problem for the end user. The polymers themselves are essentially non-biodegradable. Furthermore, specific organic inverting surfactants must be used with these polymers to enable them to be properly hydrated and dispersed in water. Some of these polymers have also demonstrated a sensitivity to water quality. Of course, all of the above necessitates the use of plastic (or glass) containers; a decided disadvantage.
Finally, and perhaps most importantly, these high molecular weight synthetic polyacrylamide polymers are extremely sensitive to shear stresses. Shear stressing is caused by high pressure gradients which may be imposed on a liquid by flow controllers, turbine metering systems, pumps and, in general, pressure differentials exceeding about 40 psi such as is commonly associated with aerial spray nozzles and discharge systems. Unfortunately, shear stressing damages shear-sensitive visco-elastic polymers such as the polyacrylamides by a phenomenon known as physical shear degradation. This degradation of the polymer realizes a significant decrease in solution viscosity which results in a lessening of the droplet-size distribution control effects.
In summary, the polyacrylamide drift-reducing products have several major characteristics that are not conducive to ease of use or reliable efficiency: slow hydration, water quality sensitivity and, most importantly, shear sensitivity.